Method for the production of a metal casting mould having a riser and a cavity former and riser sleeve for use therein

ABSTRACT

A cavity in a body of particulate moulding material has at least one rib-like formation for gripping an object received in the cavity. In the case of a metal casting mould a riser sleeve (1) is inserted into a cavity (9) in the top part of the mould and the cavity (9) is produced by means of a tapered former (5) the exterior of which has a plurality of radially spaced apart recesses (6) extending between the top and towards the bottom of the former and which has a negative taper from bottom to top. The depth of the recesses (6) is such that circles geometrically inscribed inside the base of the recesses (6) have diameters smaller than the corresponding mean outside diameters less three times the standard deviation of the riser sleeves (1) to be used. The recesses (6) form ribs (11) from the moulding sand and the ribs (11) grip the outer surface of the sleeve (1).

The present invention is concerned with a method for the production of ametal casting mould having a riser, and a cavity former and riser foruse therein.

The use of riser sleeves in metal casting moulds is well known. Up tonow, they have been located either by moulding directly on the patternused to make the mould or subsequently by insertion into the top or copepart of the mould into a cavity formed by a loose pattern piece whichhas been removed from the top of the mould.

It has also been proposed to insert riser sleeves into open half mouldswhere the mould has a vertical parting line.

Because of the increasing automation of methods used for the productionof casting moulds from moulding material such as sand, there is lessaccess to the pattern plates at the moulding station, especially in theproduction of repetition castings. As a result, it is no longer possibleto apply riser sleeves directly on the pattern plate, nor is it possibleto locate a loose piece on the pattern plate to form a cavity into whichis later inserted a riser sleeve.

When it is no longer possible to obtain access to the pattern plate analternative procedure may be possible with automatic moulding plantswhich are equipped with a cope mould line which is synchronised with adrag line where cores are set into the drag. On the moving cope line,inverted cope moulds are accessible for additional work, so risersleeves may be inserted in the inverted cope mould.

With riser sleeves which have been used hitherto this is eitherimpossible or only partially successful. Known riser sleeves have acylindrical or almost cylindrical outer surface for reasons partlyconcerned with their production technique and partly concerned withtheir function. Because of dimensional variations inherent in theirmethod of production such riser sleeves cannot be inserted into apreformed cavity with sufficient confidence that they will remain inplace securely.

Another group of riser sleeves, especially those which are closed bymeans of a cap at one end, have a positive taper from their base goingup towards the cap, i.e. the outside diameter becomes larger from bottomto top and therefore they cannot be used for the subsequent insertioninto the inverted cope mould.

One object of this invention is the creation of a new method for theproduction of a metal casting mould made of finely divided mouldingmaterial such as moulding sand which makes it possible to insert a risersleeve readily and so that the sleeve remains securely in a preformedcavity.

According to the invention there is provided a method for the productionof a metal casting mould having a riser, in which method a riser sleeveis inserted in a cavity in the mould, the cavity being formed bylocating a cavity former in a body of particulate moulding material,compacting the material about the former, and removing the former,characterised in that the former is made oversize relative to the sleeveto be received in the cavity and has one or more recesses thereinwhereby the cavity is formed with at least one inwardly projectingrib-like formation of moulded material for gripping the sleeve whenreceived therein and the former has a negative taper from bottom to top.

When objects such as riser sleeves are made in quantity to apredetermined nominal size, i.e. height and diameter, in practice thesleeves deviate from that nominal size. Such sleeves can still be usedprovided that they can be inserted in the mould cavities which are toreceive them, and that once inserted they will remain in place.

By checking and recording the actual height and diameter of a particularnominal size of sleeve during the course of production it is possible tocalculate the standard deviation in height and diameter from the meanvalues for those parameters for that sleeve.

The invention also includes for carrying out the method a former whoseexterior has a plurality of radially spaced apart grooves extendingbetween the top and towards the bottom of the former and whose outsidediameters are larger than the corresponding mean outside diameters plusthree times the standard deviation of the riser sleeves which are to beused. In this way the outside diameter of the former is larger than theoutside diameter of the riser sleeves which are to be inserted in thecavities. Retention of the sleeves in the cavities results from the gripon the outside of the riser sleeve of the sand ribs which are mouldedfrom the recesses in the former. This results in a clamping effectbecause according to the invention the circles which can begeometrically incribed inside the base of the recesses have diameterswhich are smaller than the corresponding mean outside diameters lessthree times the standard deviation of the riser sleeves which are to beused.

According to a preferred feature of the invention the former has at itstop end a raised rim which is so dimensioned that the total height ofthe former thus constituted is greater than the mean height plus threetimes the standard deviation of the actual riser sleeves which are to beused. Thus there remains after the insertion of the riser sleeve anannular cavity into which any sand, which has been loosened during theinsertion of the riser sleeve, may fall, without in any way hinderingthe desired depth of location of the riser sleeve.

According to another preferred feature of the invention the former hasat its top end an annular depression which is so dimensioned that theheight of the former up to the base of the annular depression is smallerthan the mean height less three times the standard deviation of theactual riser sleeves which are to be used.

By using such a former there is formed after moulding an annular rib ofmoulding material. This rib is of such depth that it touches even thelowest or shortest inserted sleeve at its top end and when longer risersleeves are inserted the pad becomes partially compressed. A sealingmeans is achieved with this arrangement.

A further example of a former according to the invention ischaracterised in that the annular depression adjoins a surface lyingabove the level of the annular depression whereby the height of theformer up to this surface is greater than the mean height plus threetimes the standard deviation of the actual riser sleeves which are to beused. By this means, using riser sleeves which are open at the top, anadditional riser volume is achieved, while when using a riser sleevewith a closed top an air gap remains between the surface and the top ofthe cap of the riser sleeve.

The invention also includes a riser sleeve of exothermic, exothermic andheat-insulating or heat-insulating material. According to the inventionsuch a riser sleeve is characterised in that its outer surface exhibitsfrom its base to its top a negative taper having an angle from 2° to 20°with respect to the vertical. Preferably, the inner and outer surface ofthe tapered riser sleeve are parallel one with another. In anotherembodiment the sleeve also has a height to diameter ratio (with respectto the lowermost internal diameter) in the range 1:1 to 1.6:1.

According to a further embodiment a riser sleeve according to theinvention is characterised in that it has a closed top in which a blindrecess is provided which reduces the thickness of the top and whichpermits the controlled venting of the riser.

Because the cavity is oversized by a predetermined amount, when a risersleeve is inserted, several air cavities are formed which are separatedfrom each other by the vertical sand ribs and which are connected onewith another by the gap at the bottom of the cavity. This has severaladvantages. Firstly, heat transfer to surrounding moulding material ormoulding sand is reduced, and the modulus extension factor of the risersleeve is increased. Secondly, the atmospheric oxygen contained in theair cavities can lead to a reduction in the necessary quantity ofoxidising agents in the sleeve material or by retaining the same amountof oxidising agents the ignition and burning rates of the exothermicsleeves can be increased. Further, in the case of green sand moulds thetransfer of moisture to the riser sleeve is reduced. Thereby theignition and burning characteristics in the case of exothermic sleeveswill become more consistent.

Preferably a breaker core is present at the lower end of the risersleeve and the core has an outside diameter larger than that of theriser sleeve so that the cavity formed by the former can be sealed bythe projecting edge of the breaker core. By choosing suitable dimensionsfor the breaker core the breaker core can be made to seal the cavityformed by the former.

The former of the invention may be made from any suitable material whichwill retain its shape during mould production.

The invention is illustrated with reference to the drawings in which

FIG. 1 is a cross section through a closed tapered riser sleeve fittedwith a breaker core.

FIG. 2 is a section through a tapered open riser sleeve fitted with abreaker core.

FIG. 3 is a top plan view of a former.

FIG. 4 is a section along the line A-B of FIG. 3.

FIG. 5 shows schematically a riser sleeve fitted with a breaker coreinserted into a casting mould.

FIG. 6 shows schematically a riser sleeve without a breaker coreinserted into a casting mould.

FIG. 7 shows a cavity in a sand mould for the later insertion of a risersleeve the cavity being formed by a former according to the invention.

FIG. 8 is analogous to FIG. 7 but shows a riser sleeve fitted with abreaker core inserted into the cavity in the sand mould.

In FIG. 1 a riser sleeve 1 has a closed top 2 and a breaker core 4fitted by glueing, whose outside diameter is larger than the outsidediameter of the riser sleeve 1. As a result the breaker core 4 exhibitswith respect to the riser sleeve 1 a projecting rim 4'.

In FIG. 2 a riser sleeve 1a is shown which is open at the top and towhich a breaker core 4 can be fitted as is indicated in therepresentation of FIG. 2.

The riser sleeve 1 or 1a is made from exothermic, exothermic andheat-insulating or heat-insulating material. The breaker cores 4 aremade of refractory material.

As can be seen from FIGS. 1 and 2 the outer surface of the respectiveriser sleeve has a negative taper from bottom to top having an angle offrom 2° to 20° with respect to the vertical. Moreover the inner andouter surfaces of the tapered riser sleeve are parallel one to another.

The ratio of height to diameter of the riser sleeve 1 or 1a lies in therange of 1:1 to 1.6:1 with respect to the lowermost inside diameter.

In the example according to FIG. 1 the riser sleeve 1 has a closed top 2and in this closed top at least one blind recess 11 which reduces thethickness of the top 2 is provided. By means of one or more of suchblank recesses a controlled venting of the riser is possible.

In FIGS. 3 and 4 a former 5 is shown whereby the left half of FIG. 4represents a sectioned view of line A-B in FIG. 3 and the right half ofFIG. 4 represents the external view.

The former of FIGS. 3 and 4 has a tapered form corresponding to that ofthe riser sleeves 1 and 1a according to FIGS. 1 and 2, but, the outsidediameters of the former 5 are larger than the respective mean outsidediameters plus three times the standard deviation of the riser sleeveswhich are to be used. By this means it is ensured that the former 5 hasat any point on its height or length an outside diameter which is largerthan the outside diameter of the riser sleeve which after moulding willbe inserted or pushed into the cavity formed by the use of the former 5.

The outer surface of the former 5 has distributed around itscircumference, several recesses 6 which extend from the top of theformer towards the bottom. The recesses 6 form ribs from the mouldingsand. The depth of the recesses 6 is chosen such that the circles whichcan be geometrically inscribed inside the base of the recesses havediameters smaller than the corresponding mean outside diameters lessthree times the standard deviation of the riser sleeves which are toused. In this way the height of the sand ribs formed in the cavity bythe recesses 6 is always such that a riser sleeve inserted into thecavity is gripped by the ribs. The riser sleeve is held fast by the sandribs with sufficient force to withstand rough treatment likely toseparate the sleeve from the cavity.

A raised rim 7 is present on top of the former 5 and is dimensioned sothat the total height of the former 5 so formed is larger than the meanheight plus three times the standard deviation of the riser sleeveswhich are to be used. By "mean height" of the riser sleeve the meanheight of a particular type of riser sleeve is to be understood.Therefore the former 5 has an added length or is oversize with respectto the length or height of the riser sleeve which is to be inserted. Asa result there remains after the insertion of the riser sleeve an emptyannular cavity into which sand which is loosened during the insertion ofthe riser sleeve may fall. This ensures that the riser sleeve is alwaysfixed at the desired depth.

An annular depression 7a is present within the rim 7 and is sodimensioned that the height of the former 5 up to the base of theannular depression 7a is smaller than the mean height less three timesthe standard deviation of the riser sleeves 1, 1a which are to be used.As a result an annular sand pad is formed in the cavity produced by theformer 5 and the height of the pad is sufficient for the pad to contactthe top of even the shortest or lowest riser sleeve. With thisarrangement a seal is achieved especially in the case of open risersleeves, or when using capped sleeves which have been vented in thefoundry, so that when the sleeves are used and filled with metal, metalcannot flow from above, behind the sleeves.

The annular depression 7a on its inside adjoins a surface 8 which liesabove the level of the annular depression the height of the former tothis surface 8 is larger than the mean height plus three times thestandard deviation of the riser sleeves 1, 1a which are to be used. By"mean height" here again the mean height of a particular type of risersleeve is to be understood. As a result, after a riser sleeve 1 or 1ahas been inserted into the preformed cavity an air gap or a volume ofair remains between the top end of the riser sleeve and the surface ofthe cavity opposite to the sleeve.

FIG. 5 is a schematic representation showing a preformed cavity 9 formedby means of a former 5 according to the invention and in which cavity 9has been inserted a riser sleeve 1. It is to be understood that theriser sleeve 1 is in close contact on its outer surface with the ribswhich are formed on the inner surface of the preformed cavity 9 by meansof the recesses 6 in the former. In addition the cap 2 of the risersleeve 1 is in contact with the annular pad which has been formed by theannular depression 7a in the former 5. Between the outside of theinserted riser sleeve 1 and the wall of the preformed cavity 9 there areseveral air cavities separated from one another by the vertical ribs butwhich are all connected one with another by means of the annular aircavity which is formed in the preformed cavity 9 by the rim 7 on theformer 5. In addition, an air cavity exists between the cap 2 of theriser sleeve 1 and the opposite end wall of the preformed cavity 9 andis also to be seen in FIG. 5.

In the example according to FIG. 5 a riser sleeve 1 with breaker core 4is used. Since the diameter of the breaker core 4, already describedabove, is larger than the lowermost outside diameter of the riser sleeve1 a seal is formed between the projecting rim 4' of the breaker core 4and, according to FIG. 5, the lower end of the wall of the preformedcavity 9 when the riser sleeve 1 is inserted into the preformed cavity9.

FIG. 6 is an analogous view to FIG. 5 but where a riser sleeve 1, whichhas no breaker core, is inserted. In order to achieve a seal between theoutside of the riser sleeve 1 and the lower end of the wall of thepreformed cavity 9 by the use of such a riser sleeve the outsidediameter at the lower end of the former 5 is so narrowly shaped thatwhen a riser sleeve 1 is inserted a sealing contact results between theoutside of the riser sleeve and the wall of the preformed cavity 9.

FIG. 7 is a schematic view of a section of a sand mould with a preformedcavity 9 in compacted moulding sand 10 and having sand ribs 11 formed bythe recesses 6 in the former 5. An annular rib 12 is formed by the rim7, the cavity 7a and the surface 8 in the shape of the former 5.

FIG. 8 is an analogous view to FIG. 7 in which a riser sleeve 1 has beeninserted into the preformed cavity 9. It may be seen that to a certainextent during insertion of the riser sleeve 1 the sand ribs 11 have beencompressed or pinched together so that a secure fit is ensured betweenthe outside of the riser sleeve 1 and the ribs 11. 13 indicates thatpart of the sand ribs 11 with which the riser sleeve does not come intocontact. Sand 14 loosened during the insertion of the riser sleeve 1 hasfallen into the described annular cavity where it does no harm.

FIG. 8 shows once again the seal at the wall of the preformed cavity 9between the breaker core 4 and the compacted moulding sand 10.

As has been said, the riser sleeves may consist of exothermic,exothermic and heat-insulating or heat-insulating material. Examples ofsuch materials are given below:

EXAMPLE 1

Exothermic riser sleeves were made having the following composition byweight:

Silica sand--51.5%

Aluminium grindings--26.0%

Sodium cryolite--5.0%

Clay binder--2.0%

Resin binder--3.5%

Sodium nitrate--12.0%

The materials were mixed with 3-4% by weight water and compacted in coreboxes by ramming by hand or using a core shooting machine. The "green"sleeves were stripped from the core boxes and dried at 180° C. Thesleeves had a density of 1.3 g/cm³.

EXAMPLE 2

Exothermic and heat-insulating riser sleeves were made having thefollowing composition by weight:

Aluminium powder and grindings--24.0%

Boric acid--1.0%

Sodium cryolite--7.0%

Resin binder--6.9%

Iron oxide--11.0%

Silica sand--17.0%

Alumina--18.5%

Organic fibres--3.6%

Lightweight silica--11.0%

A slurry of the materials was produced in water. Formers for the sleeveswere immersed in the slurry and the solid materials in the slurry weresucked on to the formers by means of pressure. The formers were removedand the "green" riser sleeves were dewatered. The sleeves were thenstripped from the formers and dried at 180° C. The sleeves had a densityhad a density of 0.85 g/cm³.

EXAMPLE 3

Heat-insulating riser sleeves were made having the following compositionby weight:

Aluminium--5.5%

Alumina--13.0%

Resin binder--9.0%

Aluminium sulphate--1.0%

Colloidal silica sol--6.5%

Aluminium silicate fibres--6.0%

Silica fibres--55.0%

Organic fibres--4.0%

The sleeves were produced using the method described in Example 2.

The sleeves had a density of 0.45 g/cm³.

We claim:
 1. A method for the production of a metal casting mould ofparticulate material, said mould having a mould cavity to produce saidmetal casting and a riser cavity being in communication with said mouldcavity with said riser cavity permitting insertion and gripping of ariser sleeve, said riser sleeve having an opening in communication withsaid mould cavity, said riser sleeve further having an outside diametertaper such that the outside diameter of the riser sleeve adjacent themould cavity is greater than the outside diameter of the riser sleeve atthe end remote from the mould cavity, said method involving providing ariser cavity former having a taper complementary to said riser sleevetaper, said former having at least one vertically extending recess,locating said cavity former in a body of particulate moulding material,said cavity former having been made oversize relative to the risersleeve to be received in the cavity, compacting the particulate mouldingmaterial about the cavity former and removing said cavity former to formsaid riser cavity in said moulding material, and wherein said risercavity has at least one inwardly projecting vertically extendingrib-like formation of moulded material for gripping the riser sleevewhen received therein.
 2. A method according to claim 1 characterized inthat the tapered former has a plurality of radially spaced apartrecesses extending between the top and towards the bottom of the cavityformer and the outside diameters of the tapered former are larger thanthe corresponding mean outside diameters plus three times the standarddeviation of the riser sleeves which are to be used.
 3. A methodaccording to claim 2 characterized in the circles which can begeometrically inscribed inside the base of the recesses have diameterssmaller than the corresponding mean outside diameters less three timesthe standard deviation of the riser sleeves which are to be used.
 4. Amethod according to claim 2 characterized in that said cavity formerpossesses at its top end a raised rim which is so dimensioned that thetotal height of the cavity former thus constituted is greater than themean height plus three times the standard deviation of the actual risersleeves which are to be used.
 5. A method according to claim 4characterized in that it possesses at its top end an annular depressionwhich is so dimensioned that the height of the cavity former up to thebase of the annular depression is smaller than the mean height lessthree times the standard deviation of the actual riser sleeves which areto be used.
 6. A method according to claim 5 characterized in that theannular depression adjoins a surface lying above the level of theannular depression whereby the height of the cavity former up to thissurface is greater than the mean height plus three times the standarddeviation of the actual riser sleeves which are to be used.
 7. A methodaccording to claim 2 characterized in that for the use of riser sleeveswithout breaker cores, the outside diameter of the cavity former towardsits lower end is so formed that a sealing contact is achieved betweenthe lower outside diameter of said cavity, said cavity having beenformed by the cavity former in which the sleeve is to be inserted.